This invention relates to a creep-inhibiting device for use in an automotive vehicle equipped with an automatic transmission, and more particularly to a creep-inhibiting device of this kind which has improved responsiveness to stepping-on of the accelerator pedal so as to quickly recover a creep-permitting state.
In an automotive vehicle equipped with an automatic transmission having a fluid coupling such as a torque converter, the phenomenon can occur that, due to dragging torque of the torque converter, the vehicle moves forward or creeps against the driver's will when the vehicle stands with the shift lever shifted to a drive range (forward) position, creating a load on the engine. Such load on the engine, if applied at engine idle, causes a drop in the rotational speed of the engine, even resulting in engine stall. To avoid this disadvantage, the driver has to step on the accelerator pedal by an amount corresponding to the dragging torque so as to prevent a drop in the rotational speed of the engine at idle. However, this is not desirable from the viewpoint of fuel curtailment. Therefore, it is desired that when the engine is at idle, the transmission is automatically brought into a neutral state to interrupt power transmission between the engine and the driving wheels, so that the driver is not required to step on the accelerator pedal by a large amount, for economy of fuel consumption. It is known that a creep-inhibiting device manufactured for this purpose can also provide an advantage that vibrations of the vehicle body are reduced during idling operation of the engine, which is particularly effective when the device is applied to front wheel-driven vehicles.
Although creeping of the vehicle can be avoided by reducing the torque transmission capacity of a frictionally engaging element used for starting of the vehicle, i.e. a first-speed clutch, to substantially zero upon stoppage of the vehicle, the torque transmission capacity has to soon be recovered in response to the engine load, i.e. the stepping amount of the accelerator pedal, in order to start the vehicle. Particularly when the driver is required to move his vehicle in a narrow space, for instance, to park his vehicle in a space between vehicles parked in line with the street, it is desired to maintain the clutch in a slip-permitting state so that a desired small level of power transmission capacity may be obtained through the clutch, which is proportionate to a fine stepping amount of the accelerator pedal. On the other hand, when the vehicle is started with no other vehicle running ahead, it is required to shorten the slip-permitting period to a minimum possible value, so as to promptly establish complete engagement of the clutch. Otherwise the clutch becomes engaged only after the engine speed has risen to a high level, resulting in an uncomfortable shock and early wear of the clutch.
As is known, to increase the internal pressure of the first-speed clutch in good response to the stepping-on of the accelerator pedal, desirably the pressure within the clutch should not be reduced to zero even while the creep-inhibiting device is operating, but should be controlled in advance to a value as close as possible to and at the same time smaller than a pressure value above which the clutch becomes engaged against the force of a return spring provided in the clutch, hereinafter called "the engaging pressure Pe".
However, in practice, considerable variations exist in the engaging pressure Pe required between finished products due to tolerances in manufacturing the return spring and tolerances in machining parts that accommodate the return spring, making it difficult to control with accuracy the internal pressure of the first-speed clutch to a required preset value during operation of the creep-inhibiting device. If the return spring and its related parts are to be manufactured and machined with precision so as to overcome this disadvantage, the manufacturing cost will be increased.